[LeetCode] Sort List

Sort a linked list in O(n log n) time using constant space complexity
Regardless the space constrains, it can be solved recursively. Similarly with create BST from a sorted list, it can go by in-order. Solving it in iteration will be tedious.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
    ListNode *merge(ListNode*l1, ListNode *l2)
    {
        ListNode head(0);
        ListNode *p = &head;
        while(l1 && l2)
        {
            if(l1->val < l2->val)
            {
                p->next = l1;
                l1 = l1->next;
            }
            else
            {
                p->next = l2;
                l2 = l2->next;
            }
            p = p->next;
        }
        if(l2) p->next = l2;
        else p->next = l1;
        
        return head.next;
    }
    ListNode *sortList(ListNode* & head, int start, int end)
    {
        if(start == end)
        {
            ListNode *p = head;
            //Note, move head to next for the next recursive call
            head = head->next;
            p->next = NULL;
            return p;
        }
        int mid = start + (end-start)/2;
        return merge(sortList(head, start, mid),
                    sortList(head, mid+1, end));
    }
    
public:
    ListNode *sortList(ListNode* head)
    {
        if(!head) return head;
        ListNode *p = head;
        int c = 0;
        while(p->next)
        {
            c++;
            p = p->next;
        }
        return sortList(head, 0, c);
    }
};